Dental Ceramics Flashcards
key point about ceramic
all porcelain is ceramic but not all ceramic is porcelain
decorative ceramics contain
kaolin - this is a clay
hydrated aluminium silicate that is opaque which is important for appearance of final product
how to dental ceramics differ from decorative ceramics
dental ceramics need to be translucent so kaolin is removed & replaced with feldspar & silica
feldspar
potassium & sodium alumina silicate
acts as a flux
lowers fusion & softening temp of glass
is lowest fusing component & flows during firing forming a solid mass around the other components
conventional dental ceramics
feldspathic ceramics form leucite when heated to 1150-1500o
leucite = potassium aluminium silicate
this forms glass phase of ceramic & gives powder of known physical & thermal properties
no further chemical reaction is required during fabrication of restoration
powder melts together to form crown
fabrication of crown
heating leads to sintering
ceramic particles begin to fuse into a single mass
during this material contracts by about 20%
aesthetics of conventional dental ceramics (CDCs)
colour stable
smooth surface
less staining compared to others
optical properties
optical properties (5)
reflectance - how much light is reflected from a surface / optical element
translucency - allows light to pass diffusely
opacity - does not allow light to pass diffusely
transparency - allows light fully through so objects behind can be distinctly seen
opalescence - colourless / white like an opal or changing colour like an opal
chemical stability of CDC
very stable
unaffected by wide range of pH in mouth
do not take up staining from food / drink
good biocompatibility
thermal properties of CDC
similar to tooth substance
TEC similar to dentine so results in low stresses to restoration in mouth
thermal diffusivity is low (protective of remaining tooth)
dimensional stability of CDC
once fully fired it is very stable
during fabrication shrinkage can be a problem (20%)
mechanical properties of CDC
high compressive strength
high hardness - must be glazed to prevent abrasion of opposing teeth
low tensile strength
low flexural strength
low fracture toughness
these lead to failure on loading
problems with CDC
static fatigue - time dependent decrease in strength even in absence of applied load (due to hydrolysis of SiO)
surface microcracks - can occur during manufacture, finishing or occlusal wear
slow crack growth - cyclic fatigue under occlusal forces in wet environment over time
where should CDC be used
low stress areas i.e.
- only anterior crowns
- not in all ptx
- too brittle for use elsewhere
to overcome problems with CDC
- produce a strong coping resistant to fracture & cover in conventional porcelain i.e. porcelain fused alloys
- cast or press a block of harder ceramic
- mill a laboratory prepared block of ceramic
alumina core
reinforces feldspathic ceramics
prevent cracks propagating through the material causing fracture
aluminous porcelain is opaque & can only be used as a core material so is veneered with conventional feldspathic porcelain to produce final crown
not strong enough for posterior use
zirconia
zirconia powder does not sinter unless heated over 1600o
pure zirconia can crack on cooling
yttria stabilisation of zirconia
increase yttria increases translucency but decreases physical properties
if crack begins yttria presence can cause slight expansion of material to close it; due to
zirconia = monoclinic crystal but yttria = tetragonal crystal structure
this means it is hard, strong (1000MPa flexural strength) and tough so can be used a bridge framework
fabrication of zirconia crown
imp taken of prep and sent to lab where model is cast and scanned digitally
zirconia crown then veneered with feldspathic porcelain to produce final restoration
problems with zirconia cored crowns
expensive equipment required
potential for veneering crown to debond from core
inert fitting surface so cannot etch or bond
BUT
once you have equipment they are cheaper to make
fit is generally excellent
milled core crowns & bridges
zirconia
lithium disilicate
precious metal
non precious metal
titanium
composite
ceramics all have surface sintered layer for best aesthetics
fabrication of a milled crown
scan in mouth
design on CAD machine
mill
polish
cement
cast & pressed ceramics process
more like casting a metal restoration
waxed up
invested
cast from heated ingot of ceramic (1100o)
no sintering occurs as ceramic ingot is already fully condensed prior to fitting
once devested & cleaned restoration is heated to improve crystal structure producing crack inhibiting crystals
process called ceraming
cast crown can be stained
often cut back labially & veneered with appropriate feldspathic porcelains
ceramic used in cast & pressed ceramics
lithium disilicate glass
leucite reinforced glass
